This invention relates generally to digital electronic circuitry and more specifically to circuitry providing a way fo isolating digital data bus drivers from a digital data bus.
The design of present day digital systems involves greater utilization of digital data busses for information transfer. This has resulted from the economics associated with bussed rather than point-to-point interconnection architectures. Basically the bussed system uses a single electrical conduction medium (may be one or more conductors) which is timeshared to enable transmission of digital data from multiple sources to multiple destinations. This technique is in contrast to point-to-point interconnection which dedicates a single conduction medium to transfers from a single source to one or more destinations. As more and more functionally unrelated data sources timeshare a given digital data bus, the complexity of the timesharing protocol is increased. Furthermore, the probability that one of the multiple data sources will, because of malfunctions, interfere with the other users (i.e., data sources and data destinations) of the digital data bus also increases. In fact, a given data source may malfunction in such a way as to render the digital data bus totally useless to the other digital data bus users. This normally occurs because the malfunctioning data source attempts to transmit on the digital data bus at a time other than that which is assigned by the timesharing protocol. Such a malfunction may be observed as a "stuck-at" logic level (i.e., one or more conductors within the digital data bus having a constant logical level which does not change over time) or may be observed as intermittent improper signals on the digital data bus.
In either instance, it is desirable to electrically isolate the malfunctioning data source from the digital data bus to enable the other (non-malfunctioning) users of the digital data bus to utilize the digital data bus. Malfunctioning data sources may, of course, be isolated by physically disconnecting them from the digital data bus. This is undesirable because it is slow and may be inconvenient due to system packaging. Mechanical and electro-mechanical switches have been used but they tend to be unreliable, physically large, costly to implement, and slow to react. An alternative used on occasion is the use of solid state switches which are undesirable because they substantially change the electrical characteristics of the digital data bus. The present invention provides a way of providing the desired isolation without substantially changing the electrical characteristics of the digital data bus regardless of whether a given data source is or is not being isolated.